348 Prof. A. Agassiz on Palceontological 



XLIII. — On Palceontological and Embryological Development. 

 By Prof. Alexander Agassiz*. 



Since the publication of the ' Poissons Fossiles ' by Agassiz, 

 and of the ' Embryologie des Salmonidees ' by Vogt, the 

 similarity, traced by the former, between certain stages in 

 the growth of young fishes and the fossil representatives of 

 extinct members of the group has also been observed in 

 nearly every class of the animal kingdom, and the fact has 

 become a most convenient axiom in the study of palaeon- 

 tological and embryological development. This parallelism, 

 which has been on the one side a strong argument in favour 

 of design in the plan of creation, is now, with slight emenda- 

 tions, doing duty on the other as a newly discovered article of 

 faith in the new biology. 



But while, in a general way, we accept the truth of the 

 proposition that there is a remarkable parallelism between 

 the embryonic development of a group and its palseonto- 

 logical history, yet no one has attempted to demonstrate this, 

 or, rather, to show how far the parallelism extends. We 

 have, up to the present time, been satisfied with tracing the 

 general coincidence or with striking individual cases. 



The resemblance between the pupa stage of some Insects 

 and of adult Crustacea, the earlier existence of the latter, and 

 the subsequent appearance of the former, in palseontological 

 history, furnished one of the first and most natural illustra- 

 tions of this parallelism ; while theoretically the necessary 

 development of the higher tracheate insects from their early 

 branchiate aquatic ancestors seemed to form an additional 

 link in the chain, and point to the Worms, the representatives 

 of the larval condition of Insects, as a still earlier embryonic 

 stage of the Articulates. 



Indeed, there is not a single group of the animal kingdom 

 in which embryology has not played a most important part 

 in demonstrating affinities little suspected before. The deve- 

 lopment of our frogs, our salamanders, has given us the key 

 to much that was unexplained in the history of Keptiles and 

 Batrachians. The little that has been done in the embryology 

 of Birds has revolutionized our ideas of a class which at the 

 beginning of the century seemed to be the most naturally 

 circumscribed of all. Embryology and palaeontology com- 

 bined have led to the recognition of a natural classification 



* From ' Science ' for September 18, being a verbatim report of the 

 address delivered at the Meeting of the American Association for the 

 Advancement of Science at Boston, August 1880. 



